1. Field of the Invention
The present invention relates to a piezoelectric resonator of high electro-mechanical coupling comprising an X-cut plate of lithium tantalate (LiTaO.sub.3) crystal. The X-cut plate is a crystal plate which is perpendicular to the X-axis of the crystal. Such an X-cut plate includes a plate which is rotated about the Y-axis and/or Z-axis of the crystal.
2. Description of the Prior Art
Piezoelectric resonators comprise a plate of piezoelectric material such as crystal or lithium tantalate and an electrode film coated upon the upper and lower main surfaces of the plate. When an alternating (a.c.) electric field is applied between the electrode films, the piezoelectric plate is subject to stress of a frequency equal to that of the a.c. electric field. One kind of vibrating stress is called thickness shear mode. When the frequency of the a.c. electric field coincides with the inherent resonance frequency of the piezoelectric plate, electric resonance occurs, resulting in greater oscillation.
Oscillating devices comprising such piezeoelectric resonators are widely used in oscillators or filters of communication apparatus due to their compactness, high reliability, and high stability of oscillation frequency. Recently, single-mode resonators in which spurious responses are eliminated and multi-mode filters have been developed wherein the oscillation energy is trapped or concentrated in the vicinity of the electrodes by the mass load effect and the piezoelectric reaction.
In a prior art, LiTaO.sub.3 piezoelectric resonator, spurious response was generated in the vicinity of the inherent frequency of the resonator, resulting in degradation of the resonator. The quality factor Q, which corresponds to the stability of frequency of oscillation, was also low in the prior art resonator. Further, in the prior art resonator, it was attempted to minimize fluctuations of frequency with respect to the change of temperature so as to obtain stable temperature characteristics by selection of the cutting direction of the piezoelectric plate with respect to the crystal orientation. However, it is not easy to determine the optimal cutting direction and it is difficult to obtain a single mode resonator in which spurious responses are eliminated.